Production of metal-ceramic substrates which have on one or both surface sides of a ceramic layer one metal layer each, for example, a copper layer, by means of the so-called direct bonding technique (DCB technique or process) which is also called the direct copper bonding technique or DCB technique, in metal layers of copper is known. In this process described, for example, in U.S. Pat. No. 3,744,120 or DE-PS 23 19 854 the bond is produced between the ceramic layer and a metal foil or board which forms the respective metal layer by the fact that the board on its surfaces has a layer or a coating (fused-on layer) of a chemical compound consisting of the metal and a reactive gas, preferably oxygen, that after placing the metal foil or board on one surface side of the ceramic layer this arrangement of ceramic and metal is heated to a process temperature which is above the eutectic temperature of the chemical bond between the metal and reactive gas, but below the melting point of the metal so that a liquid phase is formed on the outside surface of the metal foil or board (on the fused-on layer) via which then the bond with the ceramic takes place.
In electrical circuit engineering through connections which establish electrical connections, for example, between printed circuits, contact surfaces etc. from two different sides of a substrate are often required.
These through connections are common, for example, in circuit board engineering and are produced there, for example, by chemical and/or galvanic means by copper or other suitable materials. This technique is limited to relatively low currents and is little suited in particular to power circuits with high currents.
To produce printed circuits use of conductive or silk screen printing pastes which contain a metal and a binder and which form printed circuits from the substrate after application to a substrate and after burning-in at a temperature between 600.degree.-900.degree. C. is known from hybrid engineering (DE-PS 34 34 449). These pastes can also be pressed into connection channels or windows in the substrate during application so that in this way through connections are obtained.
The disadvantage here is the low conductivity of the electrical printed circuits and connections or through connections produced with these pastes. These conductive pastes used in hybrid engineering must be burned-in generally in air. These pastes are therefore not suitable for producing metal-ceramic substrates using the DCB technique, since the DCB process must be carried out in a protective gas atmosphere. Furthermore, the coefficient of thermal expansion of a burned-in conductive or silk-screen printing paste differs significantly from the coefficient of thermal expansion of a massive metal layer. Since metal-ceramic substrates are used for electrical power circuits or modules and are exposed there to very frequent cyclic temperature loads with extreme temperature differences these silk-screen printing pastes are fundamentally unsuited for through connections in ceramic-metal substrates which are produced according to the DCB process.
Finally, a process is also known (DE-PS 41 03 294) which is designed especially for producing through connections by means of the DCB process. In this known process a metal powder, preferably a copper powder which is designed to fuse on during the DCB process and join with the metal layers is introduced into the respective opening of the ceramic layer provided for the through connection in order to form an electrically conductive bridge which links the metal layers on the two surface sides of a ceramic layer. The disadvantage in this process is especially that the desired result can only be achieved with extremely accurate process control, in particular it can be ensured only with extremely accurate process control that the desired through connection or bonding with the required cross section actually takes place also on the two metal layers. Furthermore, the disadvantage in this process is also that the effective cross section of the through connection and thus the conductivity of this connection are limited at a stipulated size of the opening or window. Since the through connection or bridge is essentially shaped like a sleeve or hollow cylinder the electrically effective cross section of this bridge is therefore only a fraction of the window cross section.